Friction lining compositions



May 31, 19 0 F. E. STEDMAN ET AL FRICTION LINING COMPOSITIONS OriginalFiled Nov. .8. 1955 2 Sheets-Sheet l INV 41V *gggg 5 4%?2;

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FRICTION LINING COMPOSITIONS Original application Nov. 8, 1955, Ser. No.545,637, now Patent No. 2,784,105, dated Mar. 5, 1957. Divided and thisapplication Jan. 25, 1957, Ser. No. 640,622

8 Claims. (Cl. 75-206) I The present invention relates to the art offriction compositions for use in clutch and brake devices, or the like.The invention is particularly useful in high kineticenergy-absorbingdevices, but also finds application in low kinetic-energy-absorbingdevices.

This a division of our copending application, Serial No. 545,637, filedNovember 8, 1955, now Patent No. 2,784,105, issued March 5, 1957, andwhich, in turn, is a continuation-in-part of our copending application,Serial No. 257,162, filed on November 19, 1951, and now abandoned.

Friction composition lining or segments may be characterized as fallinggenerally within two categories, namely, organic and inorganic. Organiclinings are almost universally used on automotive vehicles and are usedto an appreciable extent on aircraft. Inorganic friction materials(other than solid metals) have not as yet found widespread use in thebrake and clutch art, and the reason is believed to be primarily theinstability of the frictional properties over the desired Wear-life ofthe friction article. One major deficiency of prior art frictionarticles resides in the reduction of the coefficient of friction after anumber of high temperature service applications have been made, and thisobviously is undesirable because the performance is directly dependoutupon the frictional properties of the articles.

In the past, it has been considered desirable in a prime frictionmaterial used in inorganic friction articles that the material be ofabrasive character and have strength to prevent its breaking away duringservice use. Materialsconsidered to fulfill these requirements haveincluded silica, clay, carborundum, and aluminum oxide. Suifice it tosay for the present, these enumerated materials are deficient in certainrespects and are, therefore, not regarded as satisfactory.

Therefore, it is a principal object of this invention to provide afriction composition which possesses a relatively stable or desirablecoefiicient of friction throughout its wear-life on the clutch, brake,etc. A further object is to provide such a friction lining compositionwhich, by reason of resistance to high temperatures, is especiallysuited for use in heavy duty applications. A still further object is toprovide a friction composition or article in which mullite is utilizedas a principal friction-producing agent in the braking or clutchingoperation. A still further object is to provide a friction compositionwhich will maintain substantially uniform friction-producing surfacesafter repeated operations under high temperature conditions and inheavyduty applications. For example, aircraft brakes, when applied, areheated to extremely high temperatures in a matter of a few seconds, andcurrent demands are such that the capacities of many frictioncompositions have been materially exceeded. Consequently, a new, morerugged, material is needed for such conditions, and it is a furtherobject of this invention to fulfill this need.

Other objects will become apparent as the description Pro e ds.Iathedrawings. ,4 .v

Figure 1 is a perspective illustration of a friction articleincorporating the present invention;

Figure 2 is a perspective illustration of the supporting element ofFigure 1;

Figure 3 is a cross-section of a slightly different embodiment from thatshown in Figure 1;

Figure 4 is a sectional illustration of a disc brake incorporating anembodiment of this invention; and

Figure 5 is a broken, greatly enlarged, sectional detail view, showing agrain containing mullite crystals retained by a metallic binder for afriction-producing function and also showing the grain presenting newfrictional surfaces after wear.

Broadly, the product which we have invented is a friction materialcomposition or article which is suitable for use as a brake or clutchlining (or the like)'and which contains an inorganic material known asmullite and a suitable binder material, together, in some instances,with other ingredients.

We have discovered that mullite, or a frangible material containingmullite, such as calcined kyanite, etc.,

when retained by a suitable binder, is effective as a friction materialbecause the grain of mullite tends to wear under use and to present newor renewed frangible apic'es which provide effective friction-producingsurfaces- Any binder which is effective for retaining the mullite grainsin friction-producing position while allowing wear of the grains toproduce the new frangible apices, may be employed. However, where themullite grains are to be used to the maximum advantage, particularlyunder high temperature conditions or in high kinetic-energy-absorbingdevices, it is desirable to use metallic binders, which are both moreheat resistant than most other binders and more elfective in retainingthe mullite grains in the friction article.

In one form of this invention, the binder consists of a strong,heat-resistant, wear-resistant, ductile, malleable metallic substancewhich is combined with the prime friction material, identified above asmullite, so as to support the mullite in friction-producing position sothat wear of the mullite brings about renewed or new frangible apiceswithin the mullite grains. In such a friction article, copper and aheat-treated aluminum silicate in which mullite is predominant mayconstitute the principal ingredients, and an example formulationcontaining these ingredients may consist as follows in approximatepercentages by weight: Copper 16% to 86%, zinc and/or tin in amounts upto 41%,'and calcined kyanite (which is predominately mullite) 1% to 55%,

Test results indicated that with certain binders up to 70% of calcinedkyanite may be used, this being the equivalent'of about 60% of mullite.While amounts of calcined kyanite as low as 1% are believed to bebeneficial to the friction material, outstanding .advantages over otherprime friction-producing ingredients are clearly indicated by testresults of compositions using as little as 4% of calcined kyanite or 3%of mullite.

Under certain conditions, the zinc and tin may be entirely eliminated orused interchangeably in any desired proportions up to the specified 41%.Other ingredients such as nickel, cobalt, iron, and brass maybesubstituted for the copper. In addition, the composition may contain anyone or more of the constituents: lead 1% to 20%, flake graphite 1% to19%, or si1ica'1%to 20%.

assuage, r

found to produce articles suitable for brake or clutch use, the articleshaving desirable friction, wear, and strength characteristics. Thecalcined kyanite (or similar material as. explained hereafter) istheprimary friction-producing ingredient, and should be used insuflicient quantity to obtain the desired coefficient of friction. Theminimum useful'amount of calcined kyanite (or mullite) depends upon theperformance characteristics which need to be attained. Themaximum usefulamount of calcined .kyanite (or mullite) depends largely upon thecharacteristics of the matrix (or binder). i

In the form of the invention just described, the important elements aremullite and a strong metallic binder serving to retain said mullite inposition; Ineffect, the frictional composition comprises ametalliematrix having a plurality of pores distributed throughoutitsmass, and particles of mullite disposed in saidpores, said matrixserving to positively secure said mullite in positionwith respect to themass of said matrix. The proportions of mullite are not critical,themullite giving its effective friction-producing action in anyquantity used. The pri-,

mary function of the binder metal or other material is to form aroundthe mullite grains and mechanically hold them in positionso thatfracture occurs within the grains and without-pulling-the grains out ofthe binder material. This is. best accomplished by employing aplastically deformable binderwhich retains the mullite with sufiicienttenacity so that Wear of the'friction composition creates new frangibleapices-within the mullite grains and without fracture-between the binderand the grains.

As stated, 'use of the calcined kyanite (or mullite) is largelyresponsible for the excellent wear-resistance of our improved frictionlining; Therefore, it is necessary to use, enough calcined kyanite (ormullite) to obtain the desired low wear rate. Obviously, if the'amountof ceramic material (and other non-binder constituents) is too high, theamount of metallic binder (for example, copper) will be insufficient toprevent crumbling of the lining, i.e., the lining becomes deficientinsofar as internal strength is concerned. This need for an adequateamount of binding (or matrix) material'is the primary limiting factordetermining the usable amount 'of ceramic material. Different bindingmaterials, because of their variant characteristics asbinders, permitdifferent maximum amounts of ceramic material to be incorporated in thelining.

The optimum ratio of metallic matrix to primary friction-producingceramic (calcined-kyanite or mullite) is dependent upon the .particularusage of the friction material. Once a particular usage of the frictionmaterial is selected, .it then becomes possible to select the ratio ofmatrix to ceramic which is best adapted to the conditions'encountered inthe desired application The upper limit of ceramic is determined by theability of the metallic matrix to form a cohesive compact. Physicalproperties such as wear rate arid effectiveness (orcoetficient offriction) are influenced by a change in the ceramic'content.Assuming'that the proposed usage ofthe material dictates a high ceramiccontent, then such a material can be fabricated, the only limitationbeing that the matrix must have sufficient strength to retain theceramic material in position under operating conditions. a

It has been discovered that the various matrix ingredients differ intheir capacity to hold the material together. Since the matrixingredients also have an influence on the effectiveness-and wear rate ofthe material, the selection of the matrixingredients is both on thebasis of the amount of ceramic required and on the basis of the frictionand 'wear characteristics of the matrix itself. It is, therefore,apparent that the choice of the ceramicto-matrix ratio isa matter ofdesign preference. The formulating of friction materials for specificuses proceeds on the theory that there is no given formulation which issuperior at all performance conditions.

The st1:en gth of; the article must be sufficient :to. .Withstand theshear loads encountered by conventional friction lining materials andmust be sufficient to prevent undue tearing and falling away withsuccessive service applications. The strength is primarily due to themetallic binder. While the use of insufficient metallic binder reducesthe strength of the lining, the use of excessive metallic binder tendsto produce a fast-wearing product. The amount and constituency of thebinder should not under any circumstances be such as to suppress thefrictional characteristics of the mullite or dominate the frictionalproperties of the lining. a

The zinc and/ or tin maybe used to change the friction characteristicsof the matrix by alloying with it and also may be used aslubricants-when present as unalloyed constituents. If used in excessiveamounts, they tend to produce a low-melting article having obviouslimitations for service use.

Since carbonaceous lubricants tend to detract from matrix strength, inthose instances where high concentrations of ceramic are desired thegraphite brother-carbonaceous lubricant may be entirely eliminated.There is, of course, a loss'in whatever effectis. provided by thelubricant, but'this can be compensated.forf-through appropriate matrixformulation. Also, the particular appli? cation, for example, clutches,may dictate a high concert trationof ceramic and have no requirement fora lubricant. i

The finished article, for practical purposes, may be a solid homogeneousmass or mixture of the various elements which is embodied in a flat,disc-shaped form for convenient assembly. The friction article may, ineffect, be substituted for the conventional organic friction lining usedin a brake or clutchfor frictional engagement with a relativelyrotatable member.

The source of mullite-producing ceramic and the amount thereof used inthe present invention may vary somewhat depending upon the friction andwear propertiesdesired. This material serves as the primary friction,-producing ingredient and is preferably dispersed uniformly throughoutthe article.

Mullite is a mineral having a formula 3Al O -2SiO Mullite may be formedby heating combinations of alumina (A1 0 and silica (SiO to about 1000C. or higher.

It crystallizes in the orthorhombic system and is characterized'by lathor needle-like crystals having an almost square cross section. Mullitedoes not occur as a natural mineral in commercial quantities, but iscustomarily derived from other materials by heat treating processeswhich are fully explained hereafter.

Mullite is commonly derived for commercial use in one of two ways. Thefirst method is to fuse the; proper stoichiometric proportions ofalumina andsilica in an electric furnace. If the alumina and silicausedin this process are of a high purity-and theproper; amounts are used, apure form of mullite (containing less than 1% ,of other ingredients) canbe obtained. a

The second method is to calcine any one of three natural-occurringaluminum silicates at the proper temperature. These three minerals arecommonly called: kyanite, sillimanite, and andalusite, and form a tri-vrnorphic series having the-chemical formula Al,O .SiO,. When any of theminerals are heated to the propcrtem-i perature, mullite is formedaccording to the "following reaction 3(Al O .SiO- plus Heat=3Al O .2SiOI plus SiO The silica '(SiO liberated in, this reaction is either ahighly siliceous glass ora high temperature crystalline form of silica(cristobalite or tridymite) de-! pending on the minor impuritiesof theraw material. It is, of course, desirable that the heat treating processbe carried on under such circumstances that'the con! version of the basematerial to mullite is as complete asthe constituency of the rawmaterial will allow.

assesses rial in which mullite is predominant. Minor amounts ofunconverted base material usually remain in the calcined alumina-silicamaterial due to a non-uniform-temperature distribution during the heattreating process.

Andalusite has a crystalline structurein the orthorhombic system andoccurs in nature as a grey, greenish or reddish mineral with a specificgravity of 3.0 to 3.5. The largest domestic source is found in MonoCounty, California. Other deposits are rare, but some have been found inNevada and New England. This material begins disassociation to mulliteat 1410 C., and the mullite conversion is substantially complete at 1500C. A slight expansion occurs when mullite is formed with little or nodisintegrating or disruptive effect.

. Kyanite has a crystalline structure in the triclinic system and has aspecific gravity of 3.5 to 3.7. The most important deposits are found inVirginia, South Carolina, Georgia, and India. This substance beginsdisassociation to mullite at 1100" C., and the mullite conversion issubstantially complete at l4l0 C. This conversion is accompanied by anexpansion of which produces multiple fractures in the kyanite grain. Thegrain does not disintegrate, and this result is believed to' be due tothe bonding effect ofthe siliceous glass in which has developed aquantity of minute crystals of tridymite and/or cristobalite. While thepercentage of mullite in the calcined kyanite varies, generally theproportion of the mullite therein is about 70% to 75% of the calcinedkyanite; therefore, the approximate equivalent of 4% of the calcinedkyanite is 3% of pure mullite. Pure calcined kyanite has a content ofabout 85% mullite.

Sillirnanite has a crystal structure in the orthorhombic system and hasa specific gravity of 3.23. Some sillimanite is found in South Dakota,but the best known deposits are in India. This material beginsdisassociation to mullite at 1550" C., and the mullite conversion issubstantially complete at 1625 C., with little or no expansion takingplace.

' In all three forms of the foregoing natural-occurring materials, theformation of the mullite crystals starts on the surface of theraw-material grains. In andalusite grains, the mullite needles orientthemselves parallel to the C or longitudinal axis of the originalcrystal. In kyanite and sillimanite, the mullite crystals grow inwardlyin a direction perpendicular to the surface.

Other natural-occurring materials, such as topaz or clays having astheir predominant constituents minerals of the kaolinite group oraluminous groups are available for making useful forms of mullite.

- Typical compositions which may be used to produce a brake or clutchfacing, or lining, are as follows (the percentages of the ingredientsbeing by weight):

Percent Lead 1 to 20 Quartz 1 to 20 Graphite 1 to 13 Calcined kyanite 4to 55 In conducting experiments directed toward increasing thepercentage of mullite in our friction material, it was found that, withcopper as the sole matrix ingredient, a cohesive and functionallysuperior friction material can be obtained with percentages of calcinedkyanite or pure mullite substantially above 55%. For example, thefollowing formulations have given satisfactory test results:

Although the use of copper as the sole matrix'ingredient has'beenmentioned as an expedient which permits the inclusion of a higherpercentage of calcined kyanite or pure mullite in the friction material,there are undoubtedly other matrix constituencies whcih would be aseifective as, or more effective than, copper in increasing the amount ofthe ceramic material which can successfully be included in thecomposition. Experience which has been accumulated in the development ofmatrix materials indicates that future efforts should continue the trendtoward increasing the usable upper limits of the ceramic frictionproducing ingredient.

The matrix ingredients are also selected on the basis of heatresistivity requirements for the finished material. Where exceedinglyhigh temperatures are encountered, more refractory, higher melting pointmetals, such as iron, nickel, and cobalt may be substituted for lowermelting point metals, such as copper. Examples of such formulations areas follows:

'The' friction articles produced 'by following the fore goingformulations possess frictional properties and wearresistance which suitthem for use in various types of kinetic-energy-absorbing devices. Eventhough extremely high I frictional heats are developed, the-coefficient:of frictionof the articles is maintained. at a satisfactory. and stablelevel throughout the. wear-life. of the articles..- The friction. andwear properties. may be. varied, forvexample, by varying the quantityvof calcined ltyanite.v Therefore, the amount to be used .will depend.upon design needsi It has been noted in the use of frictioncompositions which incorporate, as the primary friction-producingingradient, strong abrasives such as silica, aluminum oxide, etc.,. thatafter a relatively few braking stops in which high temperatures aregenerated, the braking effectiveness diminishes. This reduction isbelieved to be due to the fact that the abrasives become fire polishedat the high heats encountered, thereby substantially impairing thefrictional characteristics thereof.

Heretofore, it has been considered necessary that inorganic frictioncompositions contain strong abrasive materials, i.e., the abrasivecrystals or grains be comparatively strong and resistant to breakageunder service usage. This being true, repeated braking applicationswhich generate high friction surface temperatures do not break orfracture the grains which, therefore, tend to become fire polishedorsm'oothed, thereby seriously affecting the friction properties.

In contrast with thiscommon belief in the art, the present inventionincorporates a relatively weak and friable friction constituent in theform of mullite or the aforementioned mullite-bearing materials. Themullite provides a wear surface of frangible apices extending outwardlyfrom the face of the friction article. When fractured (or worn) theseapices reform into multiples of the original apices, thereby alwayspresenting a friction surface having a desirably high coeflicient offriction. In actual aircraft brake tests in which friction surfacetemperatures are believed to reach 1650 C. (3000 F.), the grains of thefriction-producing mullite donot appear to fire polish but rather tofracture and produce more pointed grains without formation of a glassyphase.

The following formulations are listed as specific examples of frictionlinings usable in heavy duty kineticenergy-absorbing devices. Of theseformulae, J and K represent preferred versions of our friction material.Formula I being an example of an excellent lining having a relativelysmall amount of mullite, and Formula K being an example of an excellentlining having a considerably higher concentration of mullite. Thepercentages given are by weight.

Formula I Percent Copper 66 Zinc 12 Tin 6 Iron 6. Graphite 6 Calcinedkyanite 4 "Formula Copper-lead (same as in Formula L) L.. 68 Quartz 5Graphite 1 Calcined kyanite 26 Formula L U Brass chips 29 Copper-lead 139 Quartz a 5' Graphite 7 l- Calcined kyanite 26 5 (Th1s material in.raw state is atomizedcopper-lead powder in the proportion of partscopper to 35 parts lead and will pass through'a 200 mesh screen.) V

Formula N Percent Cop 62 Zinc 1 12 Tin 6 Iron 7 Graphite 6 Relativelypure mullite 7 Formula 0 Copper 61 Zinc 12 Tin 6 Iron 8 Graphite 6Calcined andalusite 7 Formula P Copper 61 Zinc V 12 Tin 6 Iron 8Graphite 6 Calcined sillimanite 7 In general, our improved frictionmaterial is produced from finely divided or powdered ingredients whichare thoroughly mixed to a homogeneous mass and then compacted either ina shallow retaining cup or a suitable die under a relatively highpressure. The compacting pressure may be allowed to dwell on the compactfor a period of time, such as thirty (30) seconds. The compact, if madein a die, may be inserted into a reinforcing cup either before or aftersintering. In any event, the material is sintered in such a manner as tocause alloying or coalescing of the metal powders, thereby forming ahighly-porous metallic matrix with the other materials dispersedthroughout the matrix and firmly secured in place in the various pores.The finished product is then a composite of the ingredients joinedthrough various degrees of physical combinations, and is characterizedas a compact, self-supporting unit or lining.

The ingredients mixed together may have particle sizes approximating thefollowing: copper through 325 mesh, iron through 200 mesh, zinc about150 mesh, tin through 325 mesh, calcined kyanite through 10 mesh,mullite through 10 mesh, quartz about 140 mesh, lead about mesh,graphite (natural flake graphite), and brass chips through 10 mesh andon l00 mesh. These sizes may be varied considerably depending upondesign de,-

siderata; however, it has been found that the ideal grain size for thecalcined kyanite is 40 to 60 mesh. The degree of purity of theingredients may also be consider-- ably varied in accordance with designpreferences as will blender which is allowed to run forapproximatelyonehalf u o or J This resultantmix may be compacted into substan--tially solid form, and at least two methods for making a compact areavailable. In the first method, the material may. be, compressed into avshallow cup-shaped support 10 of Figure2 ofthe attached drawing. The.cupshaped support 10 is formed ofcold rolled steel and copper plated .toa thickness of approximately 0.0005 inch to 0.001 inch, the actualthickness not being critical but sufficient to serve the purposehereinafter described. This copper-plated cup is then placed into anintimately fitting die and a measured amount of mix poured therein andsmoothed over to provide fairly uniform thickness and density throughoutthe powdered mass. Next a suitable flat-faced. plunger, brought .to.bear on the to 100,000 pounds 9 exposed powder surface with a pressureof about 40,000 per square inch, thereby compacting the material intothe cup.

, The compacted product now obtained is a unified mass of controlledporosity and density retained in substantially solid form by means ofthe pressure and intimate contact between the ingredient particles.

Next the compact is sintered, preferably in a reducing atmosphere, at atemperature of approximately 1100 F. to 1900 F. for a period of abouttwenty minutes to one and one-half hours. The reducing atmosphereprevents oxidation in the mass and makes possible metal-tometal contactof the various metal particles to facilitate coalescence or secureadherence of such particles together. This result is believed to occurby reason of the chemical combination of the reducing atmosphere withthe metal oxides, thereby leaving pure metal surfaces in direct cohesiveand/or adhesive contact. The sintering temperature must, of course, bekept below the melting point of the copper or other predominant metalsince it has been found that once the metal flows, 1t separates from theother ingredients. The purpose of sintering is to keep the metalparticles in substantially the same relative compacted physicalpositions, but at the same time cause direct coalescence therebetween toform an integral metal-alloy mass or matrix having irregularly dispersedpores which are filled with the other compact ingredients. Coalescenceor alloying is facilitated by the use of zinc and tin, which in meltingwet the particles of copper and promote alloying. The re sultant porousmatrix serves as a mechanical binder for securing the variousnon-metallic or non-alloying particles in place.

It should be here stated'that there are two reasons for copper platingthe retaining cup, the first being to provide an interface between thecup and compact which serves as a joining or bonding medium therebetweenin brazing the two bodies together. This interface is shown as the darkline 12 in Figure 3 and under microscopic examination blends into thematerials of both bodies. This bond, in conjunction with the forces offriction bes eaves tween the compact and cup sides 14, rigidly securesthe parts together. The cup sides provide appreciable lateral supportagainst the compact shearing away during a service application. 7Further, as the second of the abovementioned two reasons, the copperplate protects the cup from decarbonizing or oxidizing during thesintering operation, both or either of these two reactions, if" allowedto occur, serving to weaken the bond between the cup and compact.

As mentioned earlier, there are two methods by which our improvedfriction material may be produced, the one just described beinggenerally to compact the powders into the retaining cup, then sintering.As the second method, instead of compacting in the cup proper, a diehaving a concavity substantially the equal of the cup is used to receivethe powders which are therein compacted and after removal in compactform, sintered under substantially the same conditions as before.However, the resultant article is formed with a bevelled peripheralsurface at an angle of about 75 degrees to the bottom face. Next thecompact is placed in a cup (not necessarily copper plated) and a coiningoperation performed thereon during which the cup sides are forcedagainst the peripheral surface of the compact. Thus, as seen in Figure3, a mechanical clamping arrangement is provided for securing theassembly together. The coining operation develops substantial axialpressure on the compact and cup so as to fill out any voids which mayoccur between the cup and compact or in the-compact itself, and to bringthe thickness of the over-all assembly within tolerance dimensions.

The article as illustrated in the drawings may conveniently beincorporated in a disc brake as disclosed in 'Du Bois et al. Patent2,483,362. 'A mere substitution of the article for the Du Bois et al.patent friction material is all that is necessary, and this may beaccomplished by fastening the bottom 16 of the compact of Figure 3, bywelding or the like, to one of the nonrotatable discs so that thefriction face 18 is juxtaposed with one of the rotatable brake discs.Generally speaking, wherever an organic friction lining segment is usedin disc brakes, an article of this invention may be substitutedtherefor. In certain instances, slight design changes may be necessaryin the brake to accommodate the new form of article.

In illustration of how the present invention may be adapted for use inan aircraft brake, reference is made to Figure 4 for anillustration ofsuch adaptation, the brake of this figure being closely similar 'to theone illustrated and claimed in Du Bois et al. Patent 2,483,362. In thisfigure, a wheel 20 is rotatably supported on axle 22 by means ofbearings 24. This wheel is provided with an overhanging rim portion 26which is equipped with a plurality of driving keys 28, said keysextending axially through peripheral slots 29 in rotatable discs 30, 32,34, and 36 to drive the same. The number of rotating discs may be variedaccording to the requirements of the particular brake installation.These discs are movable axially along the driving keys 28 for frictionalcontact with the cooperating nonrotating disc, members of the brakestructure.

These nonrotating disc members are supported on a fixed member 38, whichis suitably secured to axle 22. The member 38 has a nonrotatable andaxially fixed disc 40 held thereon by means of a plurality of throughbolts 42. Sleeves 44 are mounted on the bolts 42 and serve as anchorsfor four axially movable but nonrotatable discs 46, 48, 50, and 52. Bothsides of discs 48, 50, and 52 are provided with the friction articles orcompacts 54 made according to the foregoing explanation of thisinvention. Also, the left face of disc 40 and the right face of disc 46are provided with compacts 54. These compacts 54 may be used in anydesired number, and as illustrated are used in sufficient number to beequally spaced about the circumferential extents of the discs.

The actuating means for exerting compressive force on the brakediscscomprises a piston 56 which is movable axially within a chamber 58provided in the member 38. The .piston 56 and its associated chamber 58are, in the'present instance,' O-shaped.

Any means may be used to fasten the compacts 54 to the correspondingdiscs, and as illustrated, a rivet-type fastening is used. Whatever typeof connection is used, it is essential that it be of sufficient strengthto retain the compact on the "respective disc during the extreme shearloads produced by braking applications. Thus, it is possible that thebottom of the compact cups may be welded by means of a convenient'process to the disc members.

In operation, fluid under pressure is introduced into chamber 58 todrive piston 56 toward the right. Piston 56 then forcibly engagesnonrotatable disc 46 and thereby compresses all of the discs intofrictional interengagement against the backing member 40. For release ofthe brakes, the fluid pressure introduced into chamber 58 is relieved,thereby allowing disengagement of the disc members. During frictionalengagement of the discs, the friction faces 18 of compacts 54 directlyengage the rotating discs 30, 32, 34, and 36, respectively, so as toproduce the desired braking torque.

In the illustration given in Figure 5, 60 designates a grain of mulliteor calcined kyanite, the grain being formed at least principally ofmullite crystals, and 61 designates the metallic binder retaining thegrain in place for frictionproducing action. The original apices of thegrain are indicated by the numeral 62. Under the braking or clutchingoperation, the apices 62 are broken awayand renewed so that newfrictional spices 63 are formed; thus, as the composition wears inactual operation, new irregular or pointed surfaces are provided tocontinue the friction-producing operation. This characteristic of themullite crystal in fracturing under wearing stress so as to present newapices is believed to account for the desirable results described.

In the use of the present invention, it has been found that even withextended heavy duty use, the coeflicient of friction of the article willremain relatively constant throughout the wear-life thereof, and in somecases will actually increase slightly as wear progresses. Stated inother words, there is generally no tendency toward a deterioratingcoefiicient of friction as in the use of prior art friction articles. y

The terms friction article'and friction composition" as used herein meanand include, and are intended to mean and include, friction segments orlining having use in brakes, clutches, or the like devices, as one partof the principal friction-producing elements of the devices. Forexample, the composition of the present invention could be used aslining for the brake shoes in the conventional automotive vehicle drumbrake assemblies or as linings on friction elements of disc brakes. Ofcourse, the means by which the actual friction-producing product of thisinvention may be fastened in the clutch or brake assemblies may vary tosuit design requirements.

The numerous specific formulations cited in this specification aremerely examples of useful combinations of ingredients, and are notintended to detract from the breadth of the concept that constitutesapplicants invention, i.e., the concept that an improved frictionmaterial can be provided by combining suitable amounts of mullite and ametallic binder.

Although several embodiments of the invention have been illustrated anddescribed, various changes in the form and relative arrangements of theparts or ingredients may be made to suit requirements.

We claim:

1. In a process for forming a clutch or brake friction article, thesteps of calcining kyanite to form mullite in excessof 70% by weight ofthe kyanite, mixing the calcined kyanite material with a metal powderwhich is deformable under pressure to flow around said mullite andmechanically grip said kyanite material, compressing the mixture to aunified mass, sintering the mass in a protective atmosphere ata't'emperature of 1100 to 1900 F. to unit the metalparticles,and'subjecting the united mass to pressures from 40,000 to 100,000p.s.i. to fillv out the. voids therein while confining said mass againstlateral expansion. 3

2. The process of claim 1, inv which the sintering step is carried outin a reducing atmosphere.

3. The process of claim 1, in which silica is added to the frictionmaterial.

4. The process of. claim 1, inwhich the sintering operation is carriedon within a period of '20 minutes to one and one-half hours.

5. In a proces's for forming a clutch or brake friction article,thesteps of calcining kyanite at a temperature between approximately1100 and 1410 C. to form mullite in excess by weight of. 70% .of thecalcined kyanite material, mixing the calcined material with metalpowder which is deformable under pressure to flow about said calcinedkyanite material and provide a rigid me chanical interlocking therefor,compressing the material into a rigid compact, and heating the compact.to coalesce and thereby adhere the metal particles and thereby bind themullite during the wear life of the friction composition, said heatingbeing eflectedso that the structure of the metal powders coalesce insubstantially their original position and without aggregating relativelyto the other ingredients.

6. In a process for forming an integrated cup and friction compositionarticle suitable for use in a brake or clutch, the steps of mixingdeformable metal powder and an alumina-silica. material containingmullite as the prime friction ingredient, said metal powder beingdeformable under pressure to mechanically grip said alumina-silicamaterial, said mullite being in excess of 3% by weight of thecomposition, compressing the mixture to form a rigid compact, sinteringthe mixture at a temperature of .1100 to 1900 F. to unite the metalparticles, compressing the compact within said retainer cup. to fill outthe voids between said compact and said cup, and pressing the walls ofthe cup into interlocking en gagement with said compact.

7. 'In a process for forming a clutch or brake friction article, thesteps of heating alumina-silica material to form mullite in excess of70% by weight of said ma terial, crushing the said heated material andsizing said crushed material so as to pass through a 10 to mesh screen,mixing the material with a metal powder which deforms under, pressure tomechanically grip said alumina-silica material, compacting the mixtureof metal powder and alumina-silica material, sintering the metal powderabout said mullite material to unite said metal particles, andsubjecting themass to pressures of 40,000 to 100,000 p.s.i.

8. The method of claim 5, in which the sintered composition iscom-pressed under pressures of 40,000 to References Cited in the file ofthis patent UNITED STATES PATENTS 2,784,105 Stedman et al'. Mar. 5, 1957Kinzer July 9, 1895

1. IN A PROCESS FOR FORMING A CLUTCH OR BRAKE FRICTION ARTICLE, THESTEPS OF CALCINING KYANITE TO FORM MULLITE IN EXCESS OF 70% BY WEIGHT OFTHE KYANITE, MIXING THE CALCINED KYANITE MATERIAL WITH A METAL POWDERWHICH IS DEFORMABLE UNDER PRESSURE TO FLOW AROUND SAID MULLITE ANDMECHANICALLY GRIP SAID KYANITE MATERIAL, COMPRESSING THE MIXTURE TO AUNIFIED MASS, SINTERING THE MASS IN A PROTECTIVE ATMOSPHERE AT ATEMPERATURE OF 1100 TO 1900* F. TO UNITE THE METAL PARTICLES, ANDSUBJECTING THE UNITED MASS TO PRESSURES FROM 40,000 TO 100,000 P.S.I TOFILL OUT THE VOIDS THEREIN WHILE CONFINING SAID MASS AGAINST LATERALEXPANSION.